A variety of print methods have been employed for imaging various sheet materials. Commonly employed print methods include gravure, offset, flexographic, lithographic, electrographic, electrophotographic (including laser printing and xerography), ion deposition (also referred to as electron beam imaging [EBI]), magnetographics, ink jet printing, screen-printing and thermal mass transfer. More detailed information concerning such methods is available in standard printing textbooks.
One of ordinary skill in the art appreciates the differences in these various print methods and recognizes that a combination of ink and receiving substrate that results in high image quality in one printing method often exhibits an entirely different image quality with another print method. For example, in contact printing methods such as screen-printing, a blade forces the ink to advance and wet the receiving substrate. Image defects are typically due to a subsequent recession of the ink contact angle with the substrate. In the case of non-contact printing methods such as ink jet printing, the individual ink drops are merely deposited on the surface. In order to achieve good image quality, the ink drops need to spread, join together, and form a substantially uniform, leveled film. This process requires a low advancing contact angle between the ink and the substrate. For any given ink/substrate combination, the advancing contact angle is typically significantly greater than the receding contact angle. Accordingly, ink/substrate combinations that result in good image quality when printed with contact methods such as screen printing, often exhibit insufficient wetting when imaged with non-contact printing methods such as ink jet printing. Insufficient wetting results in low radial diffusion of the individual ink drops on the surface of the substrate (also referred to as “dot gain”), low color density, and banding effects (e.g. gaps between rows of drops).
Another important difference between screen-printing and ink jet printing is the physical properties of the ink. Screen printing ink compositions typically contain over 40% solids and have a viscosity of at least two orders of magnitude greater than the viscosity of ink jet printing inks. It is not generally feasible to dilute a screen printing ink to make it suitable for ink jet printing. The addition of large amounts of low viscosity diluents drastically deteriorates the ink performance and properties, particularly the durability. Further, the polymers employed in screen printing inks are typically high in molecular weight and exhibit significant elasticity. In contrast, ink jet ink compositions are typically Newtonian.
Ink jet printing is emerging as the digital printing method of choice due to its good resolution, flexibility, high speed, and affordability. Ink jet printers operate by ejecting, onto a receiving substrate, controlled patterns of closely spaced and sometimes overlapping ink droplets. By selectively regulating the pattern of ink droplets, ink jet printers can produce a wide variety of printed features, including text, graphics, holograms, and the like. The inks most commonly used in ink jet printers are water-based or solvent-based inks that typically contain about 90% organic and/or aqueous solvents. Water-based inks typically require porous substrates or substrates with special coatings that absorb water.
One problem, however, with ink jet inks is that ink compositions do not uniformly adhere to all substrates. Accordingly, the ink composition is typically modified for optimized adhesion on the substrate of interest. Further, good wetting and flow onto various substrates is controlled by the ink/substrate interaction. Preferably, the interaction results in a sufficiently low advancing contact angle of the ink on the substrate, as previously described. Accordingly, the image quality (e.g. color density and dot gain) of the same ink composition tends to vary depending on the substrate being printed.
Various approaches have been taken to improve image quality of water-based ink jet inks. For example, U.S. Pat. No. 4,781,985 relates to an ink jet transparency, which exhibits the ability to maintain the edge acuity of ink patterns or blocks of the transparency. The transparency comprises a coating thereon which includes a specific fluorosurfactant. Ink dry times are improved upon utilizing an emulsion of a water insoluble polymer and a hydrophilic polymer as the coating on the transparency. The addition of a water insoluble polymer prevents film tackiness during handling, and by reducing water receptivity slightly, allows the ink droplets to spread before the ink solvent vehicle absorption takes place.
WO 02/062894 A1 describes a coating composition comprising (a) at least one binder and (b) at least one filler having a surface areas of at least about 1 m2/g and wherein the topcoat derived from the coating composition is printable with a UV curable ink-jet ink. Also described is an article with an ink receptive printing layer, comprising a substrate having a topcoat, wherein the topcoat is printable with UV curable ink-jet inks.
EP 0 615 788 A1 (Watkins) describes a method for forming clear coats on retroreflective articles utilizing an aqueous coating composition comprising water, water-borne dispersion of polyurethane, and cross-linker, and optionally acrylic emulsion. Also described are retroreflective articles formed according to the method and a preferred liquid coating composition for use in the method and in making the articles.